Use genetic diagrams to predict the results of monohybrid crosses involving codominance or sex linkage and calculate phenotypic ratios

To predict the results of monohybrid crosses involving codominance or sex linkage, we can use genetic diagrams such as Punnett squares. Below, I will demonstrate how to set up these diagrams for both scenarios and calculate the corresponding phenotypic ratios.

1. Monohybrid Cross Involving Codominance

Example: Blood Type InheritanceIn the ABO blood group system, the alleles involved are:

• IAIA: A antigen (dominant)
• IBIB: B antigen (dominant)
• $i$: No antigen (recessive)

Cross: A person with blood type A (genotype IAiIAi) is crossed with a person with blood type B (genotype IBiIBi).

Parental Genotypes

• Parent 1: IAiIAi (Blood Type A)
• Parent 2: IBiIBi (Blood Type B)

Gametes

• Parent 1 can produce: IAIA or $i$
• Parent 2 can produce: IBIB or $i$

Punnett Square Setup

Results

• Offspring Genotypes:
  • 1 IAIBIAIB (AB)
  • 1 IAiIAi (A)
  • 1 IBiIBi (B)
  • 1 $ii$ (O)

Phenotypic Ratios

• Phenotypes:
  • 1 AB : 1 A : 1 B : 1 O
• Ratio: 1:1:1:1

2. Monohybrid Cross Involving Sex Linkage

Example: Red-Green Color BlindnessRed-green color blindness is an X-linked recessive trait. The alleles are:

• X^c: X chromosome with the allele for color blindness (recessive)
• X: Normal X chromosome

Cross: A color-blind male (genotype XcYXcY) is crossed with a carrier female (genotype XcXXcX).

Parental Genotypes

• Parent 1 (Male): XcYXcY (Color Blind)
• Parent 2 (Female): XcXXcX (Carrier)

Gametes

• Parent 1 can produce: XcXc or $Y$
• Parent 2 can produce: XcXc or $X$

Punnett Square Setup

Results

• Offspring Genotypes:
  • 2 XcXXcX (Carrier females)
  • 1 XcYXcY (Color-blind male)
  • 1 $XY$ (Normal vision male)

Phenotypic Ratios

• Phenotypes:
  • 2 Carrier females : 1 Color-blind male : 1 Normal vision male
• Ratio: 2:1:1